DISCOVERY 



33 



indicated in the Press to-day by the Admiralty for 

 assisting navigation at sea as well as in the air." 



It will be gathered from Admiral Jackson's closing 

 remarks that some of the war-time direction-finding 

 stations are still being used ; but for the more peaceful 

 purpose of assisting in the navigation of ships and 

 aircraft. The stations were not used for that purpose 

 to any great extent in the war, though directional 

 sets fitted on aircraft were used pretty frequently for 

 navigational purposes by obtaining bearings from 

 known fixed land stations. 



At first sight, it would appear that directional 

 receiving sets in ships or aircraft would be invaluable 

 for obtaining bearings from land stations, and so they 

 would be if the results obtained were always reliable. 

 There are, however, errors which are liable to arise 

 from various causes, some of which are not yet fully 

 understood, and cannot therefore be allowed for. The 

 most serious of these arise from bendings of the paths 

 of the waves by reflection and refraction due to certain 

 states of the atmosphere ; for instance, observations 

 may be made at, say, loo miles, within an accuracy of 

 I degree on nineteen days out of twenty, but on the 

 twentieth day there may be an error of as much as, 

 say, 5 degrees, due to some bending effect on the path 

 of the waves taking place between the sending and 

 receiving stations. There are also possibilities of local 

 errors due to the effects of metal-work, such as stays 

 for masts, etc., at the receiving station, as well as 

 bending effects on the waves due to the proximity of 

 cliffs, or long coast-lines in the path of the waves. 

 These local errors, since their causes are better under- 

 stood, can be eliminated, or corrected, sufficiently for 

 aU practical purposes, but it is clear that, at the best, 

 the bearings given by wireless directional methods 

 cannot yet be looked on as thoroughly reliable. The 

 point is, however, that they are sufficiently reliable 

 to be very useful when it is impossible to take observa- 

 tions by other means, due to, say, thick or foggy 

 weather. 



We might now examine briefly the principles under- 

 lying directional wireless, and in doing so let us first 

 recall the usual non-directional arrangement. At the 

 transmitting station, wireless waves are radiated in 

 all directions by oscillatory electric currents flowing 

 in an elevated system of wires, called the aerial or 

 antenna, which in its simplest form, and that first 

 used, consists of a single vertical wire. At the receiv- 

 ing station these waves produce similar, though much 

 weaker, currents in the' receiving aerial, which in its 

 simplest form consists also of a single vertical wire, 

 and there is no directional effect ; that is to say, the 

 direction from which the waves are coming has no 

 effect on the strength of the signals received. As 

 soon, however, as the aerial is bent or sloped out of 



the vertical, so does it become better for sending to, or 

 for receiving from, one direction more than any other. 

 This effect was foreshadowed in Hertz's original 

 experiments in Germany in 1887, but was not made 

 much practical use of until 1905, when Marconi com- 

 menced to use an inverted L-shaped aerial for trans- 

 mission and reception. Such aerials transmit best 

 to, and receive best from, the direction opposite to 

 that towards which the horizontal portion stretches, 

 but the effect is not very strongly marked. Aerials 

 of this description are used at many high-power 

 stations, so that the greatest possible radiation is 

 directed towards the receixang stations. 



Another form of directional aerial which is very 

 extensively used, especially for receiving stations, con- 

 sists of a vertical rectangular loop which best receives 

 waves travelling in the direction of the base of the 

 rectangle. Signals from any other direction are weaker, 

 until in the line at right angles to the best direction no 

 signals are heard. 



This effect can be easily seen from a consideration 

 of Fig. 2, where A and b are two vertical wires, and r 



B 



Fig. 2.— rectaxgui.ar i,oop aeri.ai,. 



is the receiving apparatus. If the waves are coming 

 from the direction shown by the arrow, or in the 

 exactly opposite direction, their effect on A will not be 

 the same, at the same moment, as their effect on B, 

 and some current will flow through the receiver. If, 

 however, the waves are coming in a direction at 

 right angles to this, i.e. through the paper, they 

 do produce the same effects in a and B at the same 

 moment, and no effect is produced in r. TMs is 

 clear if one considers the moment when the waves 

 produce currents of the same strength flowing down 

 A and B simultaneously, as then, so far as R is con- 

 cerned, these currents cancel one another and produce 

 no effect. This arrangement of directional aerials 

 was first introduced by S. G. Brown in this country in 

 1899. The Itahans, Bellini and Tosi, used triangular 

 loop aerials, as in Fig. 3, at right angles to one another, 

 in conjunction with a special recei\'ing circuit, in 

 which a coil is revolved by hand until signals become 

 loudest, when a pointer attached to the coil indicates 

 on a scale the direction from which the signals are being 

 received. As a matter of fact, greater accuracy is 

 obtained by noting when the coil is in the position 



